Magnetic and microscopic investigation of airborne iron oxide nanoparticles in the London Underground

Particulate matter (PM) concentration levels in the London Underground (LU) are higher than London background levels and beyond World Health Organization (WHO) defined limits. Wheel, track, and brake abrasion are the primary sources of particulate matter, producing predominantly Fe-rich particles that make the LU microenvironment particularly well suited to study using environmental magnetism. Here we combine magnetic properties, high-resolution electron microscopy, and electron tomography to characterize the structure, chemistry, and morphometric properties of LU particles in three dimensions with nanoscale resolution. Our findings show that LU PM is dominated by 5–500 nm particles of maghemite, occurring as 0.1–2 μm aggregated clusters, skewing the size-fractioned concentration of PM artificially to larger sizes when measured with traditional monitors. Magnetic properties are largely independent of the PM filter size (PM10, PM4, and PM2.5), and demonstrate the presence of superparamagnetic (< 30 nm), single-domain (30–70 nm), and vortex/pseudo-single domain (70–700 nm) signals only (i.e., no multi-domain particles > 1 µm). The oxidized nature of the particles suggests that PM exposure in the LU is dominated by resuspension of aged dust particles relative to freshly abraded, metallic particles from the wheel/track/brake system, suggesting that periodic removal of accumulated dust from underground tunnels might provide a cost-effective strategy for reducing exposure. The abundance of ultrafine particles identified here could have particularly adverse health impacts as their smaller size makes it possible to pass from lungs to the blood stream. Magnetic methods are shown to provide an accurate assessment of ultrafine PM characteristics, providing a robust route to monitoring, and potentially mitigating this hazard.


Temperature dependent magnetic measurements
We compare low-temperature (LT-SIRM10K) with room temperature (RT-SIRM300K) to quantify the contribution from SP particles that have blocking temperatures below room temperature (see Methods and materials for the equation). RT-SIRM300K for sample 511 (Operator cabin, Northern line, PM2.5) represents 23% of the LT-SIRM10K, meaning the remaining 77% of LT-SIRM is carried by SP particles at 300 K that become progressively blocked as the sample is cooled to 10 K (see Supplementary Fig.  S2). For platform samples 180487-56 (Platform 7 Jubilee line, Baker Street Station), 180487-58 and 180487-96 (Platform 4 Bakerloo line, Paddington Station) 60-64% of LT-SIRM10K is carried by SP particles (see Supplementary Fig. S2). Field-cooled and zero-field cooled (FC-ZFC) remanence warming curves and RT-SIRM warming, and cooling curves (see Supplementary Fig. S2) do not show any evidence of a Verwey transition or dampened transition, which is usually observed by a loss in remanence at temperatures 80-125 K upon warming.
The frequency-dependent susceptibility (lFD %) varies between 0 to 6.5% and can be used as an indicator of particles near the SP/SD threshold (see Supplementary Fig. S3). The in-phase (real) component of susceptibility χ′ reduces with an increase in frequency while out-of-phase component of magnetic susceptibility (imaginary), χ′′, shows a peak shift to lower temperatures with decreasing frequency (see Supplementary Fig. S3).
High temperature-dependent susceptibility measurement for sample 180487-87 (Oxford Circus, Central line W/B, PM4) showed an irreversible decrease in susceptibility between 206ºC and 460ºC is characteristic of maghemite -the fully oxidised, metastable form of magnetite that transforms to hematite irreversibly on heating above 200°C (see Supplementary Fig. S4), similar to diagnostic curves in a previous study 3 . There is no evidence of drop in susceptibility at 565ºC associated with magnetite on heating; a small increase below 565ºC on cooling is observed, suggesting the formation of small amount of magnetite at high temperatures (argon creates reducing conditions).

Room temperature magnetic granulometric measurements
Room-temperature cARM/SIRM ratio versus MDFAF is plotted in Supplementary Fig. S1. cARM/SIRM ratio has been used as an indicator for grain size variation for samples with homogeneous mineralogy 4,5 . The bulk magnetic properties lie close to the line defined by uniform-sized, noninteracting magnetite in the size range 1-7.5 µm (see Supplementary Fig. S1).

First order reversal curves (FORCs)
FORCs were measured for different localities within the LU (Fig. 1 B, C, and D) and for different PM size fractions (PM10, PM4, and PM1) and corresponding coercivity distributions (Fig. 1A) were calculated. The peak of the derivative of the backfield remanence curve (i.e., the coercivity distribution) occurs at an identical position (Bc = 65 mT) for all samples, and the distribution decays to zero by ~300 mT. The shape of the coercivity distribution is very similar in all air filter samples, with variations in the height of the peak caused by variations in the proportion of SP particles. We performed FORC Principal Component Analysis (FORC-PCA) 6 on our processed FORCs to test any 'fingerprint' variation between samples (Fig. 1G). All our PM samples lie between two identified endmembers (EM), which contain broadly similar features expressed to subtlety varying degrees. The magnetic signature of EM1, which primarily, but not exclusively, encompasses PM4 filter samples from the platform and ticket halls, exhibits: (1) an SD central ridge (particles between 30 nm and 70nm) at Bu=0 extending to >200 mT; (2) a clear vortex/pseudo-single domain (V/PSD) component (particles diameter between 70-700 nm); and (3) a vertically asymmetric signal at the origin that is consistent with the presence of superparamagnetic (SP) particles (nanoparticles <30 nm in diameter). Similar features are observed in EM2 (consisting primarily, but not exclusively, of PM2.5 and PM10 air filters from train operator cabins) but with relative greater intensity for the SP component and weaker intensity for the SD and V/PSD and signals compared to EM1.
To observe changes in FORC fingerprint at low temperatures, LT-FORCs were measured at 10 K on sample 180487-511 (Northern line, PM2.5 filter). At room temperature, the FORC diagram shows the presence of an SP signal and SD ridge extending to 200 mT. At 10 K, the coercivity shifts to a higher coercivity of 250 mT along the Bc axis and broadens significantly in the Bu axis. An SP signal at Bc = 0 is still observable, indicating the presence of SP particles (<<30 nm in diameter) with blocking temperatures below 10 K.
Remanence FORCs (remFORCs) were measured using the irregular measurement algorithm devised by 7,8 for sample 180487-58 (Baker Street, Bakerloo line, PM4 filter). The conventional FORC showed the same mix of SP, SD and V/PSD as the regular FORC measurements; the remFORC highlights both the SP and SD contributions. Here, the SP signal is isolated in the remFORC diagram near the SP to SD threshold size as that region is sensitive to viscous magnetization processes 8 , the transient FORC (tFORC) (see Supplementary Fig. S5) shows the transient hysteresis responses 9 related to vortex nucleation and annihilation processes, and highlights the lack of a clear MD signal (particles > 700 nm); the induced FORC (iFORC) diagram illustrates a noisy, but just visible, negative-positivenegative-positive (NPNP) signal that is related to vortex state or strongly interacting particles 10 (see Supplementary Fig. S5).  Figure S1. Bulk room temperature magnetic granulometric measurements presented on a classic sized-magnetite measurements and is indicative of mean grain size 4,5 . On y-axis we have plotted a ratio of room temperature anhysteretic remanent magnetization (ARM) susceptibility (χARM) normalized by saturation isothermal remanent magnetization (SIRM). On x-axis, the ARM mean destructive field (MDFARM) of each sample is plotted, which is defined when the magnetic fraction loses half of its remanent magnetization. Figure S2. Low-temperature remanent magnetization curves for four different samples from different localities within the London Underground. Room temperature saturation isothermal remanence (RT-SIRM) warming, and cooling curves (A, C, E) do not show any evidence of Verwey transition at 120-125 K or lower K, hinting the sample has had time to oxidize sufficiently to suppress the transition. Zerofield cooled, and field cooled (ZFC and FC) curves (B, D, F) also do not show any evidence for Verwey transition or dampened transition (usually seen for surface oxidized magnetite).  Figure S4. High temperature-dependent susceptibility for sample 180487-87. During heating, there is an observed susceptibility difference between temperature at 206ºC and 460ºC-diagnostic of maghemite. In cooling, a slight increase in susceptibility is observed at around 580ºC, which is probably a result of hematite being reduced to magnetite and is not a primary mineral present. Nb each element where strong signal shows that elements are present at above-average levels and EDS spectrum of cluster 3 (Fe-rich phase); (F) EDS spectrum after performing non-negative matrix factorisation (NMF) on the identified clusters removes the dominant background peak due to the Al SEM stub.